Semi-submersible platforms are known in various embodiments. The most common one has two buoyancy elements in the form of parallel pontoons from which a plurality of columns extend to support the deck structure of the platform. Reinforcing stays or trusses are usually arranged in planes extending transversally of the longitudinal direction of the pontoons. Another type of semi-submersible platform has a somewhat higher number of buoyancy elements, usually five or six, which are arranged in the cornes of a corresponding polygon. These buoyancy elements commonly have the form of an ellipsoid. A column extends upwards from each buoyancy element and these columns are interconnected by stiffening and reinforcing stays.
Such semi-submersible platforms are characterized in that a great part of the buoyancy will be situated relatively deep when the platform is in working condition and, furthermore, they are designated so as to provide a considerable hydrodynamic mass. At the same time, the surface breaking area of the platform and, consequently, the hydrostatic spring stiffness is comparatively low, so that the resonance period for heave, roll and pitch movements may be placed outside the wave excitation period range, i.e. usually above 20 seconds. The hydrodynamic forces acting on the submerged buoyancy elements and the forces acting on the surface braking columns, act in opposite directions so as to reduce the vertical wave force. The magnitude of this reduction of vertical forces is dependent on the the wave period, and completed cancellation of the potential pressure forces is obtained at a particular period. Consequently, two effects are inherent in the semi-submersible concept, namely no dynamic magnification due to wave excitaion at resonance, and deliberate use of wave cancellation for the potential pressure forces.
Said platform structures have in common that the buoyancy elements and the columns have such large cross-sectional dimensions that stiffeners, beams, bulkheads etc. must be used to brace the hull plates against the hydrostatic and hydrodynamic pressures pressures. This, of course, increases the weight and building cost of the structures. The length of the columns and the relatively large spacing between the buoyancy elements cause the columns to be subjected to high loads, particularly at the attachment points in the deck structure of the platfom. Furthermore, the dect structure must be made rigid and strong to take the corresponding large spans between the columns. This will also lead to increased weight, a circumstance which is further magnified due to the large safety margins which are necessary in platforms for offshore use.
Since the buoyancy elements are few and large, damage to one or more of these may easily bring the platform in a critical situation. Damage to strengthening stays may also be dangerous, and one has at least one example where the failure of such a stay has led to a serious wreck.